Glass- or plastic-made transparent substrates have conventionally been used for curved mirrors, back mirrors, goggles, window glasses, displays of personal computers, word processors or plasma displays, and other various displays for commercial use. There has been such a problem that, when it is tried to observe visual information such as objects, letters or figures through the transparent substrate, or when it is tried, in the case of a mirror, to observe an image reflected by a reflective layer through the transparent substrate, the inner visual information cannot be clearly seen because the surface of the transparent substrate reflects external light.
Heretofore, there have been known, as methods for preventing the transparent substrate from reflecting light, a method in which an antireflection coating is coated onto the surface of the glass or plastic substrate; and a method in which a thin MgF.sub.2 or SiO.sub.2 film having a thickness of approximately 0.1 micrometers is provided on the surface of the glass or plastic transparent substrate directly, or, when necessary, through a hard coat layer by a gas phase method such as vapor deposition, sputtering or plasma CVD.
Highly-functional thin films with high quality can be obtained by gas phase methods. However, when a high-vacuum system is employed to obtain such thin films, it is required to precisely control the atmosphere. Further, a special heater or ion-generation accelerator is needed, so that the production system is complicated, and becomes large in size. As a result, the production cost is inevitably increased. In addition, gas phase methods have such a problem that it is difficult to produce thin films having increased areas, or being complicated in shape.
On the other hand, among coating methods for forming functional thin films, the spray method has such problems that a coating liquid cannot be efficiently used and that it is difficult to control the conditions of film formation.
Further, when the dipping method or screen printing method, one of coating methods, is employed to form a functional thin film, materials for the film can be efficiently used. Moreover, these methods are advantageous when the mass-production of films and the plant cost are taken into consideration. However, functional thin films obtained by the dipping method or screen printing method are inferior to thin films obtained by a gas phase method in both function and quality.
In recent years, there has been proposed, as a method for obtaining thin films with high quality by a coating method, a method in which a dispersion prepared by dispersing inorganic or organic ultrafine particles in an acidic or alkaline aqueous solution is coated onto a substrate, and then calcined. This production method is advantageous when the mass-production of thin films and the plant cost are taken into consideration. However, a high-temperature calcination step is needed during the production process, so that a film cannot be formed on a plastic substrate by means of this method. Moreover, a film formed on a substrate cannot have sufficiently high uniformity due to the difference between the degree of shrinkage of the substrate and that of the film coated thereon. Thin films obtained by this method are thus still poor in quality when compared with thin films obtained by a gas phase method. In addition, a long time (for example, several tens of minutes or longer) is taken for the heat treatment, so that this method is unsatisfactory in productivity.
An object of the present invention is therefore to provide a highly-functional antireflection film with high quality, which can be produced inexpensively in a short time.